campaign create, validate finished with error handling
Handle errors during validation and add validation tests.
Add validations for campaign creation.
For some reason the travis config did not allow failures against ruby 2.* anymore.
Fix typo in error message.

Conflicts:

	lib/adcloud/campaign.rb
Update README and only enable request logging when in debug mode.
Add not found error and log error responses to debug log.
Extract find and create logic to entity class.
Add customer entity

* Find all
* Find one by id
* Create a new customer
* Update README
Guess api_endpoint from class name.
Remove explicit api_endpoint definitions.
Remove rubinius and ruby head from testing.
Add advertisement entity.
Handle unauthorized request repsonses
Update access token extraction due to changes in the API
Update authentication test ;)
Add product class and its attributes
Update product readme
Add topic class

The API supports fetching one or all topics. The result is slightly
transformed.
Change topic discount format to behave like the name hash.